Requirements to car exhaust emissions will be more and more stringent. Therefore, aftertreatment systems for exhaust gases from internal combustion engines have been developed.
Spark ignition internal combustion engines can be operated at or close to stoichiometric fuel/air conditions. It is common to treat the exhaust gas of stoichiometric operated engines with a three-way conversion catalyst which promotes the oxidation of unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides (NOX).
In order to reduce particulate emissions and to improve efficiency, compression ignition internal combustion engines are typically operated at lean fuel/air conditions. Lean exhaust gas cannot be treated with a three-way conversion catalyst, since the presence of oxygen prevents the reduction of NOX. Reducing the amount by which fuel injection timing is advanced from top dead centre can lead to a reduction in NOX emission, but to an increase in particulate emission. Increasing the fuel injection advance will have the opposite effect—hence there is frequently observed to be a “trade-off” between NOX and particulate emission.
In order to reduce NOX emissions of lean burn internal combustion engines, NOX reducing exhaust gas treatment systems have been developed. These NOX reducing systems typically comprise a NOX reducing catalyst.
In U.S. Pat. No. 5,412,946 for example, a NOX reducing catalyst comprising Pt on zeolite have been described. Such a catalyst promote the reduction of NOX to nitrogen in the presence of a reducing compound. It has been described in the art to use hydrocarbons, hydrogen or synthesis gas as reducing compound for this type NOX reducing catalyst.
NOX reducing systems that comprise both a deNOX catalyst and a NOX sorbent are also known in the art. For example from U.S. Pat. Nos. 5,874,057, 5,473,887 and WO 01/34950. During lean operation, NOX is absorbed from the exhaust gas; during richer operation, the sorbent is regenerated and the catalyst promotes reduction of NOX to nitrogen. It is disclosed that the exhaust gas can periodically be made richer (less oxygen) by adding fuel, hydrogen or synthesis gas to the lean exhaust gas.
An alternative method to reduce emissions of compression ignition internal combustion engines is by means of a process known in the art as fumigation. In a fumigation process, a gaseous fuel is mixed with the intake air of the engine prior to introducing the air/gaseous fuel mixture into the engine cylinder. Both diesel fuel and the air/gaseous fuel mixture are introduced into the engine. Known gaseous fuels for fumigation are for example natural gas, liquefied petroleum gas (LPG), and hydrogen gas.
Another method for reducing emissions, especially NOX, in an internal combustion engine is by exhaust gas recirculation (EGR). NOX emissions are reduced with the increase of exhaust gas recirculation. A high level of recirculation, however, can result in poor combustion. Various ways have been reported in the art by which recirculated exhaust gas may be enriched. In L. K. S Teo et al. “Hydrogen and Biodiesel Mixtures as Fuels for the Compression Ignition Engine” Proceedings of the THIESEL 2002 Conference on Thermo- and Fluid-Dynamic Processes in Diesel Engines, Birmingham, for example, the addition of hydrogen to recirculated exhaust gas has been described.
In order to ensure future emission limit values, especially for lean burn compression ignition internal combustion engines, there is a need to further reduce emissions, in particular the emission of nitrogen oxides.